Lunar and Planetary Science XXVIII
1727.PDF
VARIATIONS IN BRIGHTNESS OVER CALLISTO’S SURFACE AS MEASURED BY
THE GALILEO ULTRAVIOLET SPECTROMETER. A. R. Hendrix1, C. A. Barth1, A. L.
Lane2, C. W. Hord1, A. I. F. Stewart1, K. E. Simmons1, W. E. McClintock1, J. M. Ajello2, K. L.
Naviaux2, J. J. Aiello2, W. K. Tobiska2, S. K. Stephens2; 1Laboratory for Atmospheric and Space
Physics, University of Colorado at Boulder; hendrix@pisces.colorado.edu; 2Jet Propulsion
Laboratory, California Institute of Technology.
The Galileo UVS observed the multi-
At 87 RC from Callisto, the UVS
ring basin Asgard on Callisto at three different
performed the GLOBAL observation set, in
spatial resolutions during Galileo’s third orbit
which the slit was scanned across the surface
of Jupiter in November 1996. Asgard is an
at three latitudes centered on 150° longitude;
ancient ring structure centered at 30° N, 140°
the F-channel instantaneous field of view
W. The UVS F-channel, which covers the
(FOV) covered a 365 km x 1458 km region.
2000 Å - 3200 Å wavelength region, was used
The ASGARD observation was performed at
for these observations.
18 RC from Callisto. Three different latitudes
These data represent the first-ever
disk-resolved
ultraviolet
observations
were scanned, covering the region just south
of
of the crater structure, the lower rings of the
Callisto. This gives us the opportunity to seek
crater, and finally the central portion of the
spectral variations across the surface that may
the Asgard structure and the neighboring
indicate differences in composition or grain
Tornarsuk crater; the FOV footprint was 76 x
size.
We examine the spectral variations
304 km. At 8 RC from Callisto, the UVS
across the crater to look for evidence of
performed the RINGS observation, a scan
debris throughout the rings and surrounding
across the western rings of the Asgard crater
regions.
when the FOV footprint was 35 x 138 km.
We also look for variations in
brightness in the polar regions. We compare
From the GLOBAL data set, we have
the spectra measured throughout the Asgard
determined the albedo of the subsolar
basin to those obtained from the rest of
equatorial region near 110° longitude which
Callisto
resolved
we use as a standard to which to compare
observations of the western rings of the crater.
data from other regions on the body. This
and
to
the
highly
Lunar and Planetary Science XXVIII
1727.PDF
UVS Callisto Observations: Hendrix et al.
subsolar region has an albedo that decreases
from 0.09 at 3200 Å to 0.03 at 2200 Å.
The highest spatial resolution
observations,
the
RINGS
data
set,
In the GLOBAL data set, the FOV
measured the western rings of Asgard.
centered on 55°N, 120° W included the
We find that the rings on the western side
bright crater Burr (and possibly other
of the multi-ring basin are brighter than
craters), north of Asgard, which increased
the surrounding area. These rings are 1.8
the brightness measured. That region was
times
measured to be 1.9 times brighter than our
reference region.
subsolar reference region.
The GLOBAL
data set also reveals a region of relative
brightness around 60° S, 120° W, where the
albedo is 1.5 times brighter than our subsolar
reference region.
This increase in albedo
may indicate the presence of another bright
crater.
At the higher resolution of the
ASGARD observation set, we find that the
Asgard basin is brighter than the surrounding
area (2.7 times brighter than our subsolar
reference
region).
The
neighboring
Tornarsuk crater (just east of Asgard) is
particularly bright, 9 times brighter than the
subsolar reference region (accounting for the
fact that Tornarsuk filled approximately
35% of the FOV). We note an increase in
brightness
at
the
shorter
wavelengths
measured in the center of the Asgard basin.
This may indicate smaller particles which
preferentially
wavelengths.
scatter
light
at
shorter
brighter
than
the
equatorial